Hugo and Bart as conjoined babies
Hugo and Bart were born as conjoined twins. Dr. Hibbert performed a soul-smear test and found that the left twin was evil, which was confirmed by the left twin attacking the other. Dr. Hibbert then surgically separated the twins and told them to keep the evil twin, Hugo, locked in the attic. Meanwhile, Bart lived a normal life. Once a week, Homer and Marge gave Hugo a bucket of fish heads to eat.
Years later, Bart and Lisa heard noises coming from the attic. When they questioned their parents about it, Homer and Marge denied anything was wrong. Homer then took a bucket of fish heads up to the attic, which piqued their curiosity. When Homer and Marge were out the house next, Bart, Lisa, and Maggie went into the attic to find out what was up there. Hugo managed to break out of his chains, which scared the kids out of the attic. Hugo then escaped and seemingly left the house. When Homer and Marge returned home, they finally told Bart and Lisa all about Hugo. They then called Dr. Hibbert and went out to search for Hugo, whilst Bart stayed home to record the baseball game.
Bart quickly locked all the doors and windows, only to find that Hugo hadn't left the house after all. Hugo took Bart up to the attic with the intention of sewing them back together again. He revealed to Bart that he had been practicing on a Pigeon-Rat so he thought he could do it. As Hugo was about to start, Dr. Hibbert entered the attic and told Hugo to stop. Dr. Hibbert managed to knock Hugo out and stop him from sewing himself and Bart together. However, Dr. Hibbert then realized that Hugo was actually the right twin, not the left, so he wasn't the evil twin. After this revelation, Bart was locked in the attic and made to eat fish heads, whilst Hugo lived with the rest of the family.
Hugo is crazy, having spent his entire life locked in the attic with very little human interaction. He believed that he and Bart would be better of together again, as the separation was what ruined his life. Hugo also has terrible table manners, due to his life in solitude.